The present invention relates to reactors for the control of pollutant emissions from manufacturing facilities and more particularly, to a novel photocatalytic reactor and method for the destruction of volatile organic air-borne pollutants.
Increasing interest is steadily growing over the removal of the undesired organic contaminants from air streams. This is partially due to the fact that chemical plants and manufacturing facilities, especially petrochemical plants, increasingly emit air-borne pollutants. Air pollutants of major concern belong to three main classes: metals, organic and inorganic substances. Organic emissions represent a class of chemicals that can be produced for example during the incomplete, consumption of fuels used for heating and transportation. A specific class of organic emissions are named volatile organic compounds (VOCs). These are produced in various industrial operations such as paint drying, metal degreasing, printing and air striping units. VOC effluents cannot be vented directly from industrial and commercial sites due to their potential health hazards. The emissions must therefore be treated before released to the environment. Organic chemical species can be either totally mineralized (destroyed) or treated by absorption, adsorption, incineration, and condensation (Miller et al., 1993).
Adsorption processes involve contacting a polluted gaseous stream with activated carbon granules. The carbon granules act to adsorb the organic molecules in the gaseous stream leaving a clean air effluent stream. However, this process does not provide the complete destruction of the pollutants and instead only acts to transfer pollutants from the gaseous phase to the solid phase thus creating a solid disposal problem. In addition, this method is limited to gaseous streams with relatively low concentrations of organic molecules (Miller et al., 1993), because of the finite carbon adsorption capacity. Carbon particles also require regeneration and eventual disposal which represents a significant extra cost and difficulty to the process. Finally, this method does not suit all potential organic pollutants since not all of them have good adsorbability properties on the activated carbon particles.
Condensation, is not considered as a possible treatment approach because its potential use is well outside the limits set for organic pollutant concentrations. While incineration, whether direct or catalytic, has a very high operating cost which presents a serious burden on the users of this technology (Miller et al., 1993).
On the other hand, total or complete destruction or mineralization of the organic pollutants may be achieved naturally or using an oxidation process. Natural organic degradation is initiated by sunlight and molecular oxygen which are naturally abundant. However, this process is very slow and may take years to come to completion. As a result, new technologies are currently being considered to speed up these processes. One approach towards pollution abatement at chemical plant sites is to manage or control the xe2x80x9csource of emissionxe2x80x9d by various mechanisms such as using Advanced Oxidation Processes (AOPs). The purification of water and air using photocatalysts is one promising methodology of the so-called advanced oxidation processes.
Advanced oxidation processes are usually classified as homogeneous and heterogeneous processes. In the heterogeneous processes the surface of an illuminated semiconductor acts, at ambient temperature, as a catalyst by using band gap light as a source of solid excitation (Peral et al., 1992). On the other hand, the homogeneous process involves the UV photolysis of chemicals such as H2O2 and O3 to produce .OH radicals which are directly involved in the reaction (Bolton et al., 1995). The main principle involved in the homogeneous process is the generation of hydroxyl radical (OH). As the .OH radicals are formed, they attack the organic molecules and react with the pollutant in one of two ways. One possible path is the abstraction of a H atom forming a water molecule and another radical. Another possibility is the addition reaction which requires the addition of an xe2x88x92OH group to the pollutant molecule forming a combined pollutant .OH radical. The process continues with a series of reaction steps giving water, carbon dioxide and inorganic salts as end products.
Heterogeneous advanced oxidation treatment involves the accelerated oxidation of the desired chemicals with the help of ultra-violet light and semi-conductors acting as catalysts. This process utilizes TiO2 (anatase) as the photocatalyst due the fact that it is insoluble, non-toxic, has a powerful oxidizing ability, it can be excited with solar light and it is attachable to various types of supports. The possibilities for photocatalytic technology is very impressive given the minimum energy cost, or essentially zero energy cost when solar energy is employed for powering the photoreactors. Potential applications for photocatalytic reactors cover the degradation of a wide spectrum of impurity levels contained in the air as well as in industrial waste water and potable domestic water. Photocatalytic processes are also advantageous due to the fact that there is no chemical addition other than the catalyst. Also, catalyst recovery or regeneration is possible and energy is relatively inexpensive, renewable and environmentally friendly.
Carey et al (1976) were among the first to utilize TiO2 for the photocatalytic degradation of pollutants and reported that by using a light beam with a wave length of 365 nm it was possible to achieve degradation of chloro-organic molecules in water. Near UV irradiated TiO2 can also be applied for the photoconversion of organic air-borne pollutants (Holden et al, 1993). Various organic molecules such as alkanes, alkenes, alcohols, aldehydes and aromatics have all been found susceptible to this treatment. In the case of non-chlorinated compounds, no intermediate products have been observed with pollutants being completely converted to carbon dioxide. For chlorinated compounds, chlorine and phosgene intermediates have been observed (Holden et al, 1993). While results for photocatalytic degradation of pollutants are encouraging, several aspects of the technology, including catalyst activity, activity decay with time-on-stream and catalyst regeneration are not optimal (Luo and Ollis, 1996, Jacoby et al, 1996) and therefore a desired high level of efficiency is not achieved.
While some of the basic principles for photocatalysis are relatively well understood, suitable photoreactors for achieving high energy efficiency and complete photoconversion of intermediates have not yet been designed. Photocatalytic reactors designed for air borne pollutants involve different approaches for supporting the photocatalyst and for photoreactor configurations. The main choices reported are: a) entrapment of TiO2 in a glass mesh (Al-Ekabi et al, 1993), b) support of the TiO2 in coated tubes (Ibushki et al, 1993) and in honeycombs (Suzuki, 1993) and c) holding TiO2 in a ceramic membrane (Anderson et al, 1993). While the de signs in b) and c) are of limited applicability for large volumes of gases, the use of TiO2 embedded in a fiber glass mesh is an option that offers considerable potential. In this respect, a photocatalytic reactor based on this principle was reported by Al-Ekabi et al, (1993) which utilized several layers of an TiO2 impregnated mesh xe2x80x9cenwrappedxe2x80x9d on an emitting light source of the photoreactor. However, this method and the reactor had several intrinsic limitations such as a lack of a secure degree of TiO2 loading in the crystalline xe2x80x9canatasexe2x80x9d form and the lack of intimate or uniform contact of the evolving fluid (i.e. polluted air) with the mesh. Finally, only a very limited fraction of the immobilized TiO2 was being irradiated.
There is therefore an apparent need to develop a photocatalysis system for oxidizing impurities in a more efficient and effective system than could be previously accomplished by known prior art systems. It is therefore an object of the present invention to provide an advantageous and novel photocatalytic reactor suitable to process different types of air streams containing various amounts of volatile organic carbon pollutants in order to destroy the pollutants therein, which overcomes at least one of the problems and shortcomings encountered using known photocatalytic systems.
In accordance with the present invention, there has been developed a novel photocatalytic reactor and method for the treatment and degradation of organic air-borne pollutants. The novel photocatalytic reactor, herein referred to as the Photo-CREC-Air Reactor, is useful for air purification and utilizes TiO2/UV photo-oxidation technology. This reactor has been designed to provide a novel geometric configuration with optimal TiO2 catalyst loading and directed light distribution to yield optimal catalytic destruction of air borne pollutants. The fundamentally based novel design provides for optimal physico-chemical reactions and engineering aspects of the technology.
The novel photocatalytic reactor of the present invention has a variety of applications not only limited to the control of organic pollutant emission from manufacturing and commercial facilities, but also for the remediation of contaminated soils and groundwater for the improvement of indoor or closed system air quality and for the destruction of air borne microorganism contaminants. The novel photoreactor of the present invention has been named the Photo-CRE(trademark)-Air reactor incorporating TiO2/UV technology in a highly energy efficient system which is able to photoconvert significant amounts of pollutants with minimum light power.
According to an object of the present invention is a photocatalytic reactor for the destruction of organic air-borne pollutants, the photoreactor comprising a means for admission of a gas stream carry air-borne volatile organic pollutants into the photoreactor, a means for directing and increasing the velocity of the gas stream while simultaneously creating a suction effect, and a means for oxidizing and degrading the air-borne volatile organic pollutants within the gas stream. Preferably, the oxidizing means is positioned transversely with respect to the air stream flow.
The photoreactor may additionally include a mechanism to recirculate the treated gas stream back through the reactor. Additionally, the photoreactor may be designed to allow air streams to pass through without any photocatalytic treatment.
In accordance with another object of the present invention, there is provided a method for the destruction of organic air-borne pollutants, the method comprising the steps of circulating a gas stream having volatile organic pollutants therein through a photocatalytic reactor in which the gas stream is directed and its velocity increased while simultaneously creating a suction, to an irradiating section for degradation of the pollutants. Preferably the irradiating section comprises a transparent mesh transversely positioned with respect to the air flow which is homogeneously loaded with a catalyst for the treatment and destruction of pollutants within the gas stream.
According to another object of the present invention is a method for the preparation of a supported photocatalyst which method comprises applying a desired catalyst to a fibrous transparent mesh and fixing the catalyst to the mesh until a desired amount of homogeneously loaded catalyst is achieved.
According to another object of the present invention is a supported photocatalyst adapted for the photoxidation of organic pollutants in an air stream, the supported photocatalyst comprising a transparent fibrous mesh having several layers of fixed catalyst and containing up to 50% catalyst per gram of fibrous mesh. Preferably, the catalyst loaded mesh is supported by a perforated heated plate.
To demonstrate the performance of the reactor developed, toluene was employed as an example of a model pollutant. Performance evaluation also involved the qualitative and quantitative analysis of intermediate species and end products conducted at operating conditions representative of air treatment equipment. The photocatalytic reactor of the present invention was used to examine the effects of water vapor content, temperature, pollutant concentration on the photocatalytic oxidation rate which provided data to establish a photodegradation rate model as an aid for extrapolation and scaling up of the system for commercial applications.
The photocatalytic reactor of the present invention provides excellent oxidation and thus destruction of pollutants and thus the reactor can now be used for scale up and commercialization at industrial/residential sites.